Scandium is a high value metal, typically supplied in the form of scandium oxide. Annual world production of scandium is quite small, totalling approximately 10 tonnes per annum. The majority of scandium oxide production takes place in China, where scandium oxide is recovered as a by-product of other material processing activities.
Due to the small annual production of scandium, it is a high value material, with prices for scandium oxides ranging from $1,500 per kilogram to $7,000 per kilogram, depending upon purity.
Scandium is used as an alloying agent in aluminium alloys. Addition of scandium in amounts of up to 0.5% by weight to aluminium alloys can significantly improve the properties of the alloys. These alloys are used in aircraft manufacture, and sporting goods requiring high-strength, such as baseball bats, bicycle frames and bicycle components. Scandium is also finding use as a component used in mixed metal oxides in fuel cells. Scandium is also used in the manufacture of high intensity discharge lamps.
Scandium has typically been produced as a by-product of other metal recovery processes. For example, scandium has been produced from tungsten digestion sludge, uranium tailings, Bayer process red mud, titanium white hydrolytic solution, zircon ore, tantalum residues and niobium residues. Production of scandium products via hydrometallurgical pathways has typically been achieved using three main techniques or combinations of those techniques, these being ion exchange, solvent extraction or multistage precipitation and re-leaching to form an enriched scandium product from a feed solution containing scandium and a host of impurities.
Common final steps in these hydrometallurgical processes involve the production of a scandium hydroxide precipitate, which may be directly calcined to form a final scandium oxide product. Alternatively, the hydroxide may be an intermediate which is subsequently dissolved in acid and precipitated as scandium oxalate by the addition of oxalic acid. A number of earlier processes also use the direct precipitation of scandium oxalate by adding oxalic acid to aqueous strip liquors generated by solvent extraction and ion exchange techniques.
The step of forming scandium oxalate is known to be beneficial for scandium purification. Typically, however, some impurities originally present in the hydroxide or strip liquor also report to the scandium oxalate, thereby decreasing the purity of the final scandium oxide generated by calcination. Regardless of the prior processes employed, impurities tend to report into the final scandium product, thereby making the production of greater than 99.9% pure scandium oxide difficult to obtain.
U.S. Pat. No. 4,988,487, assigned to GTE Laboratories, Inc., describes an ion exchange method for recovering scandium values from industrial waste sludge. In this method, tungsten rich waste sludge is contacted with an acidic solution to dissolve scandium, iron and manganese into the acidic solution. The acidic solution contains a reducing agent such that Mn4+ ions are converted to Mn2+ ions. Ferric iron ions (Fe3+) are converted to ferrous ions (Fe2+). The solution is then contacted with an ion exchange resin at a pH of from 1.8 to 2.2, which results in scandium loading onto the ion exchange resin. An important step in this process is the reduction of manganese and iron ions to their respective divalent states, which minimises loading of these impurities onto the ion exchange resin and consequently minimises contamination of the final scandium oxide.
Scandium is subsequently eluted from the loaded resin using a chelating agent (diglycolic acid being a preferred chelating agent), which forms an enriched scandium strip solution. Scandium is then precipitated from the solution by adding ammonium hydroxide to increase the pH to between 7 and 9. This results in the formation of a scandium hydroxide precipitate which, despite the steps of iron and manganese reduction followed by ion exchange, is noted to be of only about 90% purity.
Ditze and Kongolo (1997) describe a process for the recovery of scandium from magnesium, aluminium and iron scraps. After various steps, a scandium loaded liquid organic solvent is formed and subsequently stripped using concentrated caustic soda solution. Stripping concurrently generates a scandium hydroxide precipitate that is separated and calcined to produce an impure scandium oxide product containing 0.5% magnesium and a 0.4% iron.
U.S. Pat. No. 5,787,332, assigned to Fansteel, Inc., describes a multi-stage process for the recovery of tantalum, niobium and scandium from waste residues. In this process, after several upstream leaching and precipitation steps, a scandium loaded liquid organic phase is formed, which is subsequently stripped with hydrofluoric acid solution to generate a strip liquor enriched in scandium. The addition of sodium hydroxide and heat is used to precipitate scandium hydroxide from the scandium enriched solution, noting that impurities including zirconium, titanium and iron are also present in the precipitate.
To assist in removal of these impurities, the precipitate is dissolved in hydrochloric acid, heated and pH adjusted to pH 4 with sodium hydroxide to precipitate zirconium, titanium and iron by hydrolysis. Following removal of these impurities, the scandium solution is treated with oxalic acid to precipitate scandium oxalate, which is filtered and calcined to produce a final scandium oxide product. Despite this process using the purification steps of solvent extraction, impurity hydrolysis and oxalate precipitation, the final scandium oxide does not exceed 99.0% purity.
U.S. Pat. No. 4,898,719, assigned to GTE Laboratories Inc., describes a liquid extraction process for the recovery of scandium. In this process, a digestion solution containing dissolved scandium and other base metals is formed. Dissolved iron is brought to the divalent state by reduction and the pH is reduced to about 2. Scandium is selectively extracted from the solution using an organic extractant consisting of thenoyltrifluoroacetone (TTA) dissolved in an aromatic solvent. The scandium is described as forming a very stable neutral chelate complex with the TTA. The TTA shows a high degree of selectivity for scandium over the divalent transition metals, alkaline earth metals, alkali metals and rare earth metals in the system. Scandium is recovered from the organic phase by stripping with an acid, followed by precipitation of scandium as a hydroxide or oxalate from the acid solution. The precipitation of scandium hydroxide is achieved by the addition of ammonium hydroxide and is noted to generate a product of 80.5% scandium oxide purity after calcination. The precipitation of scandium oxalate is achieved by the addition of oxalic acid and is noted to generate a product of 89% scandium oxide purity after calcination. The production of high purity scandium oxide, such as 99.9% pure scandium oxide, is not achieved, even in light of the use of iron reduction, selective solvent extraction and oxalate precipitation steps.
The present applicant does not concede that the prior discussed in this specification forms part of the common general knowledge in Australia all elsewhere.
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